1. Field of the Invention
The present invention is directed to a method and apparatus for modulating the radiation dose from an x-ray tube, and in particular to such a method and apparatus for modulating the radiation dose to achieve a predetermined effect or result associated with the radiation dose.
2. Description of the Prior Art and Related Applications
Computed tomography (CT) is recognized as a diagnostic procedure employing x-rays emitted from an x-ray tube with a relatively high dose. This means measures must be taken to maintain the exposure to radiation, to which the patient and attending personnel are subjected, to levels which do not represent a radiation hazard. With the introduction of spiral (helical) CT and multi-slice volume scanning techniques, new examination procedures have become available. The primary advantages of these new scanning techniques, such as section-to-section continuity, detection of small lesions, and rapid acquisition of data, have produced an increase in the number of patients which can be examined within a given time period, and thus have also produced an increase in the average dose per individual of the population. According to recent legislation (e.g. Council Directive 97/43/EURATON 1997) all doses due to medical exposure for radiological purposes, except for radiological therapeutic procedures, must be kept as low as is reasonably achievable. Dose reduction for CT purposes is strongly recommended and supported by the relevant national and international regulating authorities.
In accordance with these desires, scanning techniques are beginning to be employed in the field of CT wherein the x-ray level is adjusted dynamically during a scan. For this purpose, modulation of the radiation dose must be undertaken during the course of a scan. Dose modulation can be accomplished by adjusting the x-ray intensity during the rotation of the gantry of a CT apparatus, as a function of the gantry rotation angle and dependent on the instantaneous patient x-ray absorption at each particular tube angle position. Such techniques are described, for example, in U. S. Pat. Nos. 5,822,393 and 5,867,555 and 5,379,333. It is also known to modulate the x-ray dose dependent on the scanned body region by automatically adjusting the x-ray level during a longer spiral scan by making a compromise between the requirements for image quality and the applied dose, as described in U.S. Pat. Nos. 5,764,721 and 5,696,807 and 5,625,662.
It is also known to dynamically adjust the x-ray intensity during a scan dependent on the physiological condition of the organ under examination, such as the phase of a heart cycle in the case of cardio-dynamic scans, as described in German Application 19957083.3, corresponding to co-pending U.S. application Ser. No. 09/724,055 filed Nov. 28, 2000, and German Application P19957082.5, corresponding to co-pending U.S. application Ser. No. 09/724,057, filed Nov. 28, 2000. It is also known to dynamically adjust the x-ray intensity during gantry rotation as a function of the tube angle in order to protect the eyes of the patient in the case of a head scan, or to protect the physician""s hands in a biopsy scan, as described in the aforementioned U.S. Pat. Nos. 5,764,721 and 5,696,807 and 5,625,662.
None of the above-identified known techniques, however, adequately addresses the practical problem of the dose modulation speed which is available with conventional x-ray tubes. As described in U.S. Pat. No. 5,625,662, dose modulation is achieved in an x-ray tube used in a CT apparatus by modulating the tube current. The tube current modulation is, in turn, indirectly achieved by modulating the heating current supplied to the tube filament. Because of this, conventional x-ray tubes do not react fast enough to modulate the dose dependent on the instantaneous patient absorption during gantry rotation at fast speed. The failure to reproduce radiation peaks where the patient absorption is maximum increases the quantum noise in the measured signal, and significantly degrades the image quality.
The slow reaction of x-ray tubes to rapid changes in x-ray intensity is due to physical limitations in the filament structure, as well as the thermal inertia of the filament.
It is known from U.S. Pat. No. 5,822,393 that higher modulation speeds may be achieved using specially designed x-ray tubes, which have voltage-controlled gate electrode. This is a relatively expensive approach, however, and increases the complexity and cost of the x-ray tube and the associated electronics. It is known from the aforementioned U.S. Pat. No. 5,379,333, and co-pending U.S. application Ser. No. 09/376,361, filed Aug. 6, 1999, to artificially limit the modulation curve shape to a sinusoidally shaped template and to artificially limit the modulation index to a maximum of 50%. This approach is compatible with existing CT systems, but fails to achieve the maximum possible dose reduction because of the artificial limitation of the modulation index to 50%. Moreover, the sinusoidal template does not always match the peaks in the patient absorption profile, particularly in abdominal scans. Moreover, this technique does not achieve the maximum dose reduction, or satisfy requirements of other modulation techniques such as cardio-dynamic scans.
Another known technique is described in the article xe2x80x9cDose Reduction in CT by Anatomically Adapted Tube Current Modulation, II. Phantom Measurements,xe2x80x9d Kalender et al., Med. Phys., Vol. 26 (1999) pp. 2248-2253 to artificially limit the modulation index dependent only on the gantry rotation speed, for example, a maximum of 90% at 2 sec/rot to a maximum of 60% at 0.75 sec/rot. This known method, however, ignores the dependence of the modulation speed on the nominal tube current and on the focus size, and also is unable to achieve the maximum dose saving with conventional tubes.
Another known technique suggests the use of a pulsed x-ray tube with the x-ray pulse duration being adjusted to patient absorption. This technique, however, has a poor compatibility with the actual angle in angle triggered systems, and with time-triggered CT systems which use an integration period on the order of a few hundreds of microseconds. This technique also imposes significant requirements on the data measurement system. Moreover, the complete switching off of the x-rays during the pulse pauses may not be supported by all other sub-systems of a CT system, or may not be desired, such as in the case of cardio-dynamic scans.
All of the above-described known methods involve calculation of an initial x-ray profile which is desired to be achieved during a scan. An operating parameter of the x-ray tube, such as the tube current, is then varied during the scan in an effort to cause the x-ray tube to reproduce the desired x-ray profile. A conventional x-ray tube, however, as noted above cannot reproduce this profile because of its limited dynamic capabilities.
It is an object of the present invention to provide a method and an apparatus for modulating the radiation dose from an x-ray tube which allows a predetermined exposure effect, such as an x-ray profile, to be achieved using a conventional x-ray tube.
A further object is to provide such a method and apparatus wherein maximum dose reduction at target pixel noise is achieved by modulating the tube current in a conventional x-ray tube.
The above objects are achieved in accordance with the principles of the present invention in a method and an x-ray tube-containing apparatus, such as a computed tomography apparatus, wherein the x-ray tube has at least one variable operating parameter which, when varied, modulates the radiation dose, with a modulation speed, for x-rays produced by the x-ray tube, and wherein the x-ray tube is operated while varying the aforementioned parameter through a parameter range to generate modulation speed data, representing modulation speeds of the x-ray tube respectively for different values of the operating parameter. When an examination subject is to be subjected to an x-ray dose from the x-ray tube, an exposure effect associated with the exposure of the subject to the radiation dose is identified, this exposure effect being dependent on modulation of the radiation dose. When exposing the subject to the radiation dose from the x-ray tube, the operating parameter is varied in advance of a time at which the aforementioned exposure effect is to be achieved, according to the modulation speed data, so that the radiation dose is modulated to produce the desired exposure effect at the desired time.
The modulation speed of the x-ray tube can be measured during factory calibration procedures, and can be stored in computer tables as a function of the operating parameter. The tube""s modulation speed, moreover, can be identified as a function of multiple operating parameters. The measurements are performed at defined points over the entire operating range of the x-ray tube. When a patient is being exposed to a radiation dose from the x-ray tube, a dose modulation unit calculates the actual modulation speed by multi-point interpolation, using the information stored in the table. The dose modulation unit then corrects the ideal, initial modulation profile before supplying it to the tube current modulation unit of the apparatus. The actual modulation speed is updated at fixed periods during the exposure, such as twice per rotation in a CT apparatus, and is used to correct the modulation profile so that the maximum (nominal) x-ray intensity is reached for those exposures wherein maximum x-ray intensity and minimum quantum noise are desired. The dose modulation unit anticipates the x-ray peaks thereby increasing the temperature of the tube filament, and thus also increases the tube current, at an earlier time than would occur in the conventional methods described above. The maximum speed which is actually available can thereby be taken advantage of, to achieve a maximum dose saving (reduction) and a target (lower) pixel noise, despite the slow speed of conventional x-ray tubes.
The invention is based on investigations which have demonstrated that the modulation speed of conventional x-ray tubes is a tube parameter that behaves differently dependent on the gradient direction of the x-ray profile as well as on the operating parameters of the x-ray tube. This investigation has identified those operating parameters which particularly influence the modulation speed.
This investigation has also demonstrated that only the rising portion of the modulation speed profile changes with tube operating parameters. The speed of the rising gradient depends on tube operating parameters such as the maximum tube current which is to be reached, the modulation index and the size of the focus (in a multi-filament tube). Therefore, in accordance with the invention dedicated tables are measured and stored to provide the modulation speed as a function of the tube current to be reached, the modulation index (minimum tube current in the scan) and the focus size. The aforementioned investigations have determined that the modulation speed does not depend on the magnitude of the high voltage that is applied to the tube.
The aforementioned investigations have shown that if the rising gradient of the dose modulation profile is not corrected to take into account the actual modulation speed, the x-ray intensity does not reach the peaks (maximum intensity) at the desired time. Therefore, in a CT apparatus the projections subject to maximum absorption are not exposed enough, and higher quantum noise will degrade the reconstructed image. To avoid this result, the inventive method calculates the actual modulation speed based on scan parameters and corrects the rising gradients within the initial modulation profile so that the actually employed x-ray profile is always above the initial calculated profile. This means that in any projection the quantum is ensured to be lower than the maximum expected, but at the same time the maximum dose reduction is achieved at the target pixel noise with available conventional tubes.
The aforementioned investigation has also demonstrated that the falling gradient of the tube response is not dependent on the operating parameters, but is an exponential decay with a time constant that depends exclusively on the thermal cooling constant of the heating filament of the tube. This means that for a given type of tube, the falling gradient is invariant. Therefore, in accordance with the invention, the falling gradient of the modulation of profile does not have to be pre-corrected. If the actual falling gradient produced by the tube is lower than the prescribed profile shape, then the x-ray level will be higher than necessary for the target noise. This may increase the total dose, but will not increase the noise in the final image. Nevertheless, the dose reduction is still the maximum that can be achieved for the target pixel noise with a slow x-ray tube (i.e., an x-ray tube having a slow cooling filament).